The Origin of the Universe

What does science say about the origin of the universe?

Scientific evidence points to an origin sometime between 10 and 20 billion years ago. The Big Bang theory is universally accepted by those who do research on the development of the universe, galaxies, and stars as the cause of the origin of the universe. The Big Bang theory says that the universe has developed by expanding from a hot dense state with everything exploding away from everything else. What caused this explosion is not part of the Big Bang theory. It must be regarded as unknown at this time, although there are many ideas about the cause.

How do we know the universe is expanding?

The evidence indicates that as the Big Bang occurred, everything in the universe was an expanding mass of hot gas. As this mass of gas expanded, it cooled. Knots of matter formed that became the objects we see--including galaxies, stars, and planets such as our Earth. These objects continue to move away from one another even today, and, thus, our knowledge that the universe is expanding is based on observation. Evidence for the expanding universe comes from a phenomenon referred to as the redshift. When objects move apart rapidly, the light emitted by one and received by the other changes in a specific way. The light of an object moving rapidly shifts to the red end of the visible spectrum, and a redshift occurs. In the 1920's, scientists discovered that distant galaxies are moving away from us. Moreover, the farther away galaxies are, the faster the motion is relative to us and the greater the observed redshift. It happens that this kind of expansion is predicted by Einstein's general theory of relativity. This aspect of Einstein's theory has been tested by a number of experiments, and no test has been able to falsify the hypothesis. Over the years, many other scientific hypotheses have been introduced to explain the redshift without invoking an expanding universe. None of these hypotheses has a simple and direct connection with effects we can measure in the laboratory. It also is true that nearly all of them make other kinds of predictions about light that are not observed, so these hypotheses are no longer taken seriously.

Is there any other evidence for the Big Bang?

Three other major pieces of evidence indicate that the Big Bang occurred. The first is called the cosmic microwave background radiation (CMBR), which is a weak form of radiation that comes from the sky and is energy that is left over from the very early universe (fig. 14). The CMBR was predicted during World War II, and in the 1960's it was detected for the first time. Since then, it has been measured and remeasured and now ranks as the most precise scientific measurement ever made. The second major piece of evidence concerns the fraction of various kinds of atoms in the universe. Scientists have calculated the amount of helium and other light atoms that should have been formed in the first few minutes after the Big Bang. The predictions agree remarkably well with what is observed. The third piece of evidence comes from our own eyes. Telescopes currently in use in Kansas and elsewhere allow us to see faraway galaxies as they appeared close to the time of the Big Bang because light takes so long to reach us from such distant objects. The observations continue to fit our interpretation of a universe that was very different early in its history. At the present time, we do not understand everything about the development of the universe. The work of science is not finished. We do not yet know how it started or what the dark matter is. We are, however, very confident that, in general, the Big Bang model is correct, and many physicists and astronomers are now working to fill in the details.

Fig. 14--The cosmic microwave background is the afterglow radiation left over from the Big Bang. Shown here are cosmological fluctuations in the microwave background temperature made by the Cosmic Background Explorer (COBE) satellite (Spergel et al., 1999). Although extremely uniform all over the sky, tiny temperature variations can offer great insight into the origin, development, and initial structure of the universe.

Where did the Earth come from?

Most of the matter in the universe consists of such light elements as hydrogen and helium, plus an additional kind of unknown cold dark matter that is not yet well understood. Such heavier elements as carbon, oxygen, and silicon that are needed to form rocks and living organisms formed in earlier generations of stars that exploded, scattering the elements across the galaxy. These elements, sometimes referred to as ashes, were part of the matter that clumped together to form our solar system. Planets like our Earth are made primarily of the heavier elements. The Earth is known to be about 4.5 billion years old; the universe is at least three times older. A lot had to happen before the Earth could form!

What will happen to the universe in the future?

Scientists believe there are two possible scenarios. One is that the universe may collapse again into sort of a reverse of the Big Bang. The other is that it may continue to expand forever, eventually growing cold and dark. At present, the weight of evidence seems to indicate that it will expand forever. Our understanding of the nature of the cold dark matter, a subject being actively investigated by many scientists, may help us answer the question of the ultimate fate of the universe. If cold dark matter is sufficiently abundant, it could halt and possibly even reverse the universe's expansion.

Acknowledgments

We would like to thank Keith Miller, Charles Higginson, and Helen Alexander for comments on earlier versions of this booklet. Thanks go also to The Kansas Citizens for Science.

Darwin, C., and Wallace, A.R., 1858, On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection: Journal of the Proceedings of the Linnean Society of London, Zoology, v. 3, p. 53-62.

Zallinger, R.F., 1989, The age of reptiles: Peabody Museum of Natural History, mural.

Glossary

Adaptation--A trait that is particularly suited to an environment. It is the result of natural selection.

Big Bang Theory--The most supported explanation for the formation of the universe. All matter and energy in the universe came from a condensed hot mass that exploded and expanded in all directions.

Cambrian Explosion--An important event in the history of life that began around 540 million years ago and concluded around 510 million years ago. During this interval nearly all the major types of organisms now known on Earth, as well as several novel extinct types, appeared in the fossil record.

Cosmic Microwave Background Radiation (CMBR)--Radiation left over from the Big Bang. Fluctuation in the distribution of this energy is evidence of the structure of the universe right after the Big Bang.

Dark Matter--Invisible material in the universe that may mean that the universe is sufficiently heavy that it will not expand forever.

Ediacaran--A term used to describe the earliest known multicellular organisms, which appeared about 600 million years ago and largely went extinct just before the Cambrian explosion.

Evolution--Evolution is change through time. Biological evolution means change that has accompanied descent from a common ancestor.

Fossils--Any evidence of past life preserved in rocks.

Geologic Era--A long interval of geologic time recognized by the origination and extinction of a large number of plant and animal species. The boundaries usually correspond to times of major environmental change that lead to extinction and also spur evolutionary change. An example is the Mesozoic Era, when the large terrestrial dinosaurs lived. The average duration of an era is on the order of tens to hundreds of millions of years. We currently are in the Cenozoic Era, which began 65 million years ago.

Geologic Period--A long interval of geologic time, but shorter than an era, and again, defined by the origination or extinction of a large number of species. The number of species that evolve and go extinct at period boundaries, however, is less than at era boundaries. The boundaries also usually correspond to times of major environmental change, although not as major as those changes occurring at era boundaries. An example is the Cambrian Period, when animal life first appears in abundance in the fossil record. The average duration of a period is on the order of tens of millions of years. We currently are in the Quaternary Period which began 10,000 years ago.

Hominids--A group of primates that includes humans and several extinct species such as Homo erectus and the Neanderthals that all shared an upright posture.

Hypothesis--An explanation for observed phenomena. A hypothesis is used as a basis for further observations or experiments.

Law--A scientific statement that always applies, such as the law of gravity.

Macroevolution--Evolutionary changes that involve the production of new species. These occur over time scales of thousands of years. They are produced by a series of microevolutionary changes, but not all microevolutionary changes lead to macroevolution.

Microevolution--Evolutionary changes that occur within species. These occur over a range of time scales, from months to millions of years.

Natural Selection--Greater survival or reproductive success among some members of a population due to inherited traits that confer an advantage in the environment in which the population lived.

Radioactive Isotopes--Chemical elements that differ in their number of neutrons. For any radioactive isotope, the rate of decay into other elements is constant and therefore can be used to measure geologic time.

Red Shift--Light from an object that is moving away from an observer is shifted toward the red or longer wavelength end of the spectum relative to the light emitted at the source of the object. The fact that stars inside and outside of our galaxy predominantly show a red shift is evidence that the universe is expanding.

Species--A group of organisms that can interbreed with each other and produce fertile offspring. It is the fundamental unit of biological evolution.

Theory--An explanation for natural events that is based on a large number of observations and has been tested repeatedly.